1. Field of the Invention
The present invention relates to an apparatus for concentrating a water-soluble organic material, particularly, to an apparatus for concentrating a water-soluble organic material, which comprises a distillation column for distilling aqueous solutions of water-soluble organic materials which form azeotropes with water, for instance, alcohols such as ethanol, propanol, etc.
2. Prior Art
In the case of distilling an aqueous solution of 3 mol % of ethanol, for instance, distillation is conventionally carried out by using such a concentrating apparatus as shown in FIG. 7. In FIG. 7, a liquid containing ethanol (feed) is supplied to a central part of a distillation column 1. Part of the liquid in the distillation column 1 is sent from the column bottom 1a to a reboiler 2, in which it is heated to generate a vapor, which is returned to the bottom part of the distillation column 1 and goes upward in the column. The vapor and the liquid reach equilibrium and are separated upward and downward at each stage of the distillation column 1. The vapor reaching the column top 1b is cooled and condensed by a condenser 3, and refluxed to the column top 1b. As a result, the vapor and the liquid are circulated upward and downward in the distillation column 1 as shown by the arrows in FIG. 7. Concentrated ethanol is withdrawn from the column top 1b to the outside, and the residue liquid with a reduced concentration of ethanol is withdrawn from the column bottom 1a to the outside. Among paths connecting parts constituting the concentrating apparatus in the Figures described below, those shown by solid lines represent flow paths of liquid, and those shown by dotted lines represent flow paths of vapor. The arrows represent the directions of fluids flowing in the flow paths.
In the distillation column 1, for instance, concentration proceeds through the stages shown in FIG. 8. 1, 2, 3, . . . and 13, and 2′, 3′, . . . and 8′ in the Figure represent the ethanol contents in a vapor phase and a liquid phase in the stages (theoretical stages) of the distillation column 1. 1, 2, 3, . . . represent the composition at each stage in the concentrating section, and 2′, 3′, . . . represent the composition at each stage in the recovery section.
The feed and the liquid flowing downward from the stage just above are in contact with the vapor coming upward from the stage just below in the feeding section (feed stage), resulting in the compositions of the vapor phase and the liquid phase as shown in 1 in the Figure. The relation in a vapor-liquid composition between a certain stage and its above and below stages is determined by the amounts of the circulating vapor and liquid, and the amounts of the liquid and the vapor withdrawn from the column bottom 1a and the column top 1b to the outside, which is shown as the operating line in the Figure.
In the concentrating section (upper stages other than the supply stage), the concentration of ethanol changes from the vapor-liquid composition 1 at the supply stage to the vapor-liquid composition 13 at the column top 1b stepwise between the operating line and the vapor-liquid equilibrium curve. In the recovery section (lower stages than the supply stage), on the other hand, the concentration of ethanol decreases to the vapor-liquid composition 8′ at the column bottom 1 a stepwise between the operating line and the equilibrium curve.
The operating line shown in FIG. 8 represents a case where, with respect to 1 kg·mol of ethanol, 6.7 kg·mol of a vapor is generated in the reboiler 2, and 5.5 kg·mol of a vapor is refluxed by the condenser 3, so that 99% of ethanol is recovered from the feed, thereby obtaining ethanol in a concentration of 84 mol % from the column top 1b. At this time, the energy necessary for concentration is 6000 kJ per 1 kg of ethanol. Though the number of stages necessary for the concentrating section in the distillation column 1 is theoretically 13 in the example shown in FIG. 8, the distillation column is actually provided with as many stages as about 3 times that number, taking stage efficiency into account.
Though decrease in vaporizing and refluxing leads to the reduction of energy consumption in the distillation column 1, as shown in FIG. 9, reduction of vaporizing and refluxing result in the operating line with a small gradient in the recovering section and a large gradient in the concentrating section.
In the case of the concentrating operation shown in FIG. 8, the operating line becomes close to the vapor-liquid equilibrium line as the vapor-liquid composition nears the azeotropic point of water and ethanol at the column top, necessitating many stages. Accordingly, when one tries to reduce energy consumption while maintaining the vapor-liquid composition at the column top, the operating line becomes closer to the vapor-liquid equilibrium line, resulting in an increase in the stages to a practically impossible number.